One of the major factors controlling the rate of potassium secretion is the potassium content of renal tubular cells. This factor is evidently particularly important in the alterations in potassium excretion that occur in acid-base disturbances. Since little attention has been focused on the processes of potassium exchange between tubular cells and blood and how they are controlled. To investigate these processes and how acid-base factors affect them the following experiments will be performed. The isolated, perfused, bullfrog kidney will be used. Potassium and acid excretion will be measured by clearance techniques. The rate of potassium influx into tubular cells from the portal circulation will be measured by perfusing wth K42 for a set period of time and then analyzing the tissue for its content of isotope. The tissue content of potassium will also be measured. The acid-base composition of the perfusing solutions will be changed by altering the PCO2 and the concentrations of bicarbonate and other buffers. Analysis of the data should allow us to determine whether changes in K secretion in the various situations is the result of changes in influx or efflux of K across the peritubular membrane or the result of changes in the driving force for K movement out of the cell into the tubular fluid.